1. Field of the Invention
This invention relates to an exhaust gas purifying catalyst for an internal combustion engine and a process for producing the same.
2. Discussion of the Background
An exhaust gas purifying catalyst for an internal combustion engine, especially for an automobile, must have an extremely high performance in its durability and purifying ability. A monolithic catalyst and a granular catalyst have been used for the purpose.
As for the catalyst component, at least one noble metal, such as platinum (Pt), rhodium (Rh) and palladium (PD) is, carried on a catalyst support. After installing the catalyst support carrying the catalyst component thereon to a catalyst converter and letting exhaust gas go through, harmful materials like hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx) contained in the exhaust gas are purified by oxidation and reduction reactions.
The catalyst is called a 3-way catalyst, since it can purify HC, CO and NOx. A conventional catalyst has been known in which rhodium, an oxide of a rare earth element, and a metal of platinum group elements other than rhodium are dispersed on alumina grains. An as disclosed in Japanese Unexamined Patent Pulbication (KOKAI) No. 11147/1986, a catalyst is proposed in which rhodium is dispersed on alumina grains free from oxide of a rare earth element. Further, as disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 130230/1981, a catalyst is proposed which contains palladium with cerium oxide added. Furthermore, as disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 54730/1985, a catalyst is proposed in which cerium oxide is added to a 3-way catalyst having palladium as a major component.
In addition, the catalyst components are uniformly carried by the alumina grains in the conventional 3-way catalyst produced by the process described below. In the process for producing the conventional 3-way catalyst, a monolithic catalyst support made of cordierite or the like is first coated with alumina grains. And the monolithic catalyst support coated with the alumina grains is immersed into aqueous solutions of the catalyst component, such as salts of an oxide of a rare earth element, a chloride of rhodium and metals of platinum group elements other than rhodium. And then, the monolithic catalyst support coated with the alumina grains is dried to have the fine grains of the catalyst components carried on the alumina grains. The conventional 3-way catalyst thus produced is used as an exhaust gas purifying catalyst.